Modern semiconductor devices usually include multilayer structures or other periodic structures. For example, thin films of dielectric materials (e.g., polymer, oxide) and conducting materials (e.g., metal) are used in a range of microelectronic, optical, and biomedical devices. For example, GaN-based light emitting diodes (LEDs) often contain periodic superlattice structures made of alternating GaN and InGaN layers. Irregularities in these multilayer structures, such as deviations from expected periodicity and irregular interfaces between adjacent layers, can modify the electrical and mechanical properties of the structures, thereby affecting the performance of the resulting devices. Accordingly, it is desirable to monitor and control the dimensions of these multilayer structures more precisely.
One conventional approach to measuring the dimension of a multilayer structure is via imaging techniques. For example, in scanning electron microscopy (SEM), an electron beam is focused onto a small spot on a sample structure. Electrons that are scattered from the sample surface are detected to produce an image of the sample. This technique can have high contrast but is usually time consuming. In addition, the measurement is typically conduced in a high vacuum chamber, thereby increasing the cost and complexity of the measurement.
In another approach, a femtosecond laser is employed to generate acoustic waves in a periodic multilayer stack. In this case, the acoustic signal generated in the structure is periodic in time, with the period T depending on the spatial period d of the multilayer stack. Therefore, the average period of the multilayer stack can be derived from the frequency f=1/T of acoustic oscillations. However, the frequency of the acoustic oscillations may not reveal any irregularities or non-uniformities in the multilayer structure, thereby limiting its use in quality assurance of multilayer structures.